About 600 million people lost power in India on Tuesday when the country’s northern and eastern electricity grids failed, crippling the country for a second consecutive day.

The outage stopped hundreds of trains in their tracks, darkened traffic lights, shuttered the Delhi Metro and left nearly everyone — the police, water utilities, private businesses and citizens — without electricity. About half of India’s population of 1.2 billion people was without power.

India, however, has an unofficial power grid in the form of huge numbers of backup diesel generators and other private power sources. That kept electricity flowing in everything from private residences and small and large businesses to hospitals and major airports.